Screen resolution should not be identical to window size
TorBrowser design doc https://www.torproject.org/projects/torbrowser/design/ states re screen resolution: "report that the desktop is only as big as the inner content window"
Indeed, when I go to http://browserspy.dk/screen.php with TorBrowser, I get a screen size of 1057x909.
This is an obvious way to find out Tor users, and also allows to track them even better than without this change. A screen (!) resolution of 1057x909 must really stand out and allow to track a user easily. This would stay the same across all sessions and sites and even reboots.
A window/content resolution of 1057x909 isn't very common either and problematic, but that problem exists independent of the screen resolution. This issue isn't theoretical: I heard about 5 years ago that even Google tracks users based on non-standard window resolution.
That said, a screen resolution of 1057x909 would appear only among TorBrowser users and thus be fairly unique world-wide.